Method for forming silicon-oxide-nitride-oxide-silicon (sonos) memory cell

ABSTRACT

A silicon-oxide-nitride-oxide-silicon (SONOS) memory cell includes a memory gate, a dielectric layer, two charge trapping layers and two selective gates. The memory gate is disposed on a substrate. The two charge trapping layers are at two ends of the dielectric layer, and the charge trapping layers and the dielectric layer are sandwiched by the substrate and the memory gate. The two selective gates are disposed at two opposite sides of the memory gate, thereby constituting a two bit memory cell. The present invention also provides a method of forming said silicon-oxide-nitride-oxide-silicon (SONOS) memory cell.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 17/177,164, filed on Feb. 16, 2021. The content of the application is incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to a memory cell and forming method thereof, and more specifically to a two bit silicon-oxide-nitride-oxide-silicon (SONOS) memory cell and forming method thereof.

2. Description of the Prior Art

Memory is divided into two categories: volatile memory and non-volatile memory. In nowadays, the two important types of volatile memory are static random access memory (SRAM) and dynamic random access memory (DRAM). There are many types of non-volatile memory. Flash memory is the most popular type, and other types may include silicon-oxide-nitride-oxide-silicon (SONOS), ferroelectric random access memory (FRAM), phase-change random access memory (PRAM), magnetoresistive access memory (MRAM) and resistive random access memory (RRAM).

SUMMARY OF THE INVENTION

The present invention provides a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell and forming method thereof, which includes two charge trapping layers at two ends of a dielectric layer sandwiched by a substrate and a memory gate to constitute a two bit memory cell.

The present invention provides a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell including a memory gate, a dielectric layer, two charge trapping layers and two selective gates. The memory gate is disposed on a substrate. The two charge trapping layers are at two ends of the dielectric layer, and the charge trapping layers and the dielectric layer are sandwiched by the substrate and the memory gate. The two selective gates are disposed at two opposite sides of the memory gate, thereby constituting a two bit memory cell.

The present invention provides a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell including the following steps. A first dielectric layer and a first electrode layer are deposited on a substrate sequentially. The first electrode layer and the first dielectric layer are patterned, to form a second dielectric layer and an electrode layer stacked from bottom to top. Two ends of the second dielectric layer are removed, so that a dielectric layer being formed, wherein the electrode layer protrudes from the dielectric layer. Two charge trapping layers are formed on sidewalls of the dielectric layer and the electrode layer, so that the charge trapping layers have L-shaped cross-sectional profiles. Two selective gates are formed at two opposite sides of the charge trapping layers, thereby constituting a two bit memory cell, wherein FN tunneling is induced while erasing.

According to the above, the present invention provides a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell and forming method thereof, which includes a memory gate disposed on a substrate, two charge trapping layers at two ends of a dielectric layer sandwiched by the substrate and the memory gate, and two selective gates disposed at two opposite sides of the memory gate, to constitute a two bit memory cell. This device has high injection efficiency and low current consumption during programming. Sensing ability is kept while erasing, and threshold voltage is stable while programming and erasing.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 2 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 3 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 4 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 5 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 6 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 7 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 8 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 9 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 10 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

FIG. 11 schematically depicts a cross-sectional view of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention.

DETAILED DESCRIPTION

FIGS. 1-11 schematically depict cross-sectional views of a method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to an embodiment of the present invention. As shown in FIG. 1, a substrate 110 is provided. The substrate 110 may be a semiconductor substrate such as a silicon substrate, a silicon containing substrate, a III-V group-on-silicon (such as GaN-on-silicon) substrate, a graphene-on-silicon substrate or a silicon-on-insulator (SOI) substrate. The substrate 110 may be divided into a first area A and a second area B, depending upon their functions. In this embodiment, the first area A is a memory cell area while the second area B is a logic area, wherein the circuits in the logic area are used for operating the memory cells in the memory cell area, but it is not limited thereto.

Furthermore, a first dielectric layer 5 may be deposited on the substrate 110, and then an isolation structure 10 is formed in the substrate 110, wherein the first dielectric layer 5 may be a pad oxide layer, and the isolation structure 10 may be a shallow trench isolation (STI) structure, which is formed by a shallow trench isolation (STI) process, but it is not restricted thereto. Optionally, a deep well 22 in the first area A and a logic Vt well 24 in the second area B are formed in the substrate 110, and then a cell Vt well 26 is implanted in the first area A in this case.

As shown in FIG. 2, the second area B may be divided into an IO area B1 and a core area B2. The first dielectric layer 5 in the IO area B1 and the core area B2 are replaced by an IO dielectric layer 5 a and a core dielectric layer 5 b respectively, and a first dielectric layer 5 c remains. The IO dielectric layer 5 a and the core dielectric layer 5 b may be also oxides, which may be formed by different methods. For example, the first dielectric layer 5 in the IO area B1 and the core area B2 is removed, and then the IO dielectric layer 5 a may be deposited or thermally growth on the substrate 110 of the IO area B1 and the core area B2 first, and the IO dielectric layer 5 a in the core area B2 is removed, and then the core dielectric layer 5 b in the core area B2 is deposited or thermally growth, but it is not limited thereto.

As shown in FIG. 3, a first electrode layer 120 and a hard mask layer 130 are deposited blanketly in the memory area A, the IO area B1 and the core area B2. The hard mask layer 130 is deposited for patterning the first electrode layer 120 and the first dielectric layer 5 c. In this embodiment, the first electrode layer 120 is a polysilicon layer, and the hard mask layer 130 a silicon nitride layer, but it is not limited thereto. Thus, the hard mask layer 130, the first electrode layer 120 and the first dielectric layer 5 c are patterned using methods such as photolithography and etching processes, so that a second dielectric layer 5 d, an electrode layer 120 a and a hard mask layer 130 a stacked from bottom to top in the first area A, and the IO dielectric layer 5 a, the core dielectric layer 5 b, an electrode layer 120 b and a hard mask layer 130 b stacked in the second area B are formed, as shown in FIG. 4.

Two ends of the second dielectric layer 5 d are removed, so that a dielectric layer 5 e being formed, thereby the electrode layer 120 a protruding from the dielectric layer 5 e, as shown in FIG. 5. In a preferred embodiment, the two ends of the second dielectric layer 5 d are removed by wet etching, but the present invention is not restricted thereto.

Please refer to FIGS. 6-7, two charge trapping layers 140 a are formed on sidewalls of the dielectric layer 5 e, the electrode layer 120 a and the hard mask layer 130 a. More precisely, as shown in FIG. 6, a charge trapping material layer 140 conformally covers the substrate 110, the dielectric layer 5 e, the electrode layer 120 a, the hard mask layer 130 a in the first area A and the electrode layer 120 b and the hard mask layer 130 b in the second area B. Then, a part of the charge trapping material layer 140 covering the substrate 110, a top surface of the hard mask layer 130 a, and a top surface of the hard mask layer 130 b is removed to form the charge trapping layers 140 a on the sidewalls of the dielectric layer 5 e, the electrode layer 120 a and the hard mask layer 130 a, as shown in FIG. 7.

Since the electrode layer 120 a protrudes from the dielectric layer 5 e, the charge trapping layers 140 a have L-shaped cross-sectional profiles. Besides, each of the L-shape cross-sectional profile has a bottom part 140 a 1 and a vertical part 140 a 2. Due to an oxide layer 142 and a nitride layer 144 being deposited sequentially to conformally cover the electrode layer 120 a and the dielectric layer 5 e in this embodiment, the bottom part 140 a 1 of the L-shape cross-sectional profile has three layers composed of oxide/nitride/oxide layers (that is an oxide layer 142 a surrounds a nitride layer 144 a) while the vertical part 140 a 2 of the L-shape cross-sectional profile has two layers composed of oxide/nitride layers (that is the oxide layer 142 a sandwiched by the nitride layer 144 a and the electrode layer 120 a and the hard mask layer 130 a), but the present invention is not restricted thereto. Meanwhile, a charge trapping layer 140 b including an oxide layer 142 b and a nitride layer 144 b is also formed on sidewalls of the electrode layer 120 a and the hard mask layer 130 a.

As shown in FIG. 8, selective gate regions 152 are doped in the substrate 110 at the two opposite sides of the charge trapping layers 140 a. Then, two selective gates 160 including selective dielectric layers 162 and selective electrodes 164 are formed on the substrate 110 at two opposite sides of the charge trapping layers 140 a, thereby constituting a two bit (b1/b2) memory cell. In one case, the selective dielectric layer 162 may be formed on the substrate 110 by thermal oxide or chemical oxide processes, and a selective electrode layer is deposited blanketly and then patterned to form the selective electrodes 164 on the selective dielectric layer 162 beside the charge trapping layers 140 a. In this case, the selective dielectric layer 162 includes oxide, and the selective electrodes 164 include polysilicon, but it is not limited thereto.

Lightly doped source/drain regions 170 are formed in the substrate 110 beside two opposite sides of the two selective gates 160 after the selective gates 160 are formed, as shown in FIG. 9. Thereafter, the hard mask layers 130 a/130 b are removed, so that the first electrode layer 120 a and the first electrode layer 120 b are exposed. The first electrode layer 120 b in the IO area B1 and the core area B2 is patterned to form an IO gate electrode 120 b 1 in the IO area B1 and a core gate electrode 120 b 2 in the core area B2 after the selective gates 160 are formed, as shown in FIG. 10.

As shown in FIG. 11, spacers 182 are formed on the substrate 110 beside the two selective gates 160, the IO gate electrode 120 b 1 and the core gate electrode 120 b 2 at a same time in this embodiment. The spacers 182 may be respectively formed beside the two selective gates 160, the IO gate electrode 120 b 1 and the core gate electrode 120 b 2 instead. The spacers 182 may be single layer spacers or multilayer spacers, which may be composed of oxide or nitride etc, depending upon practical requirements. Source/drain regions 184 are doped in the substrate 110 beside the spacers 182. Thus, a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell 100 of two bits is carried out, and later processes may be performed, which are well known in the art and are not described. In one case, FN tunneling may be induced in the silicon-oxide-nitride-oxide-silicon (SONOS) memory cell 100 while erasing. By applying the present invention, injection efficiency is improved and low current consumption is achieved during programming, sensing ability can be kept while erasing, and threshold voltage is stable while programming and erasing.

To summarize, the present invention provides a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell and forming method thereof, which includes a memory gate disposed on a substrate, two charge trapping layers at two ends of a dielectric layer sandwiched by the substrate and the memory gate, and two selective gates disposed at two opposite sides of the memory gate, to constitute a two bit memory cell. This device has high injection efficiency and low current consumption during programming. Sensing ability is kept while erasing, and threshold voltage is stable while programming and erasing.

Furthermore, each of the charge trapping layers preferably has an L-shape cross-sectional profile, wherein a bottom part of the L-shape cross-sectional profile is sandwiched by the substrate and the memory gate for constituting a two bit memory cell, and a vertical part of the L-shape cross-sectional profile is sandwiched by the corresponding selective gate and the memory gate, for isolating the memory gate from the corresponding selective gate.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell, comprising: sequentially depositing a first dielectric layer and a first electrode layer on a substrate; patterning the first electrode layer and the first dielectric layer, to form a second dielectric layer and an electrode layer stacked from bottom to top; removing two ends of the second dielectric layer, so that a dielectric layer being formed, wherein the electrode layer protrudes from the dielectric layer; forming two charge trapping layers on sidewalls of the dielectric layer and the electrode layer, so that the charge trapping layers have L-shaped cross-sectional profiles; and forming two selective gates at two opposite sides of the charge trapping layers, thereby constituting a two bit memory cell, wherein FN tunneling is induced while erasing.
 2. The method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to claim 1, wherein steps of forming the two charge trapping layers on the sidewalls of the dielectric layer and the electrode layer comprise: depositing a charge trapping material layer conformally covering the substrate, the dielectric layer and the electrode layer; and removing a part of the charge trapping material layer covering the substrate and a top surface of the electrode layer.
 3. The method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to claim 1, further comprising: forming selective gate regions in the substrate at the two opposite sides of the charge trapping layers before the selective gates are formed.
 4. The method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to claim 1, further comprising: forming lightly doped source/drain regions in the substrate beside two opposite sides of the two selective gates after the selective gates are formed.
 5. The method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to claim 1, wherein the substrate comprises a memory area, a core area and an TO area.
 6. The method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to claim 5, wherein the first dielectric layer is deposited on the substrate of the memory area, the core area and an TO area, and then the first dielectric layer in the core area and the TO area are replaced by a core dielectric layer and an TO dielectric layer respectively.
 7. The method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to claim 5, wherein the first electrode layer is deposited on the substrate of the memory area, the core area and the TO area, and the first electrode layer in the core area and the TO area is patterned to form a core gate electrode and an TO gate electrode after the selective gates are formed.
 8. The method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to claim 7, further comprising: depositing a hard mask layer on the first electrode layer of the memory area, the core area and the TO area for patterning the first electrode layer and the first dielectric layer; and removing the hard mask layer before the first electrode layer in the core area and the TO area is patterned.
 9. The method of forming a silicon-oxide-nitride-oxide-silicon (SONOS) memory cell according to claim 7, further comprising: forming spacers on the substrate beside the two selective gates, the core gate electrode and the TO gate electrode at a same time; and forming source/drain regions in the substrate beside the spacers. 